What is the difference between synchronous motor and asynchronous motor?

Jan 7, 2021 | Blog

Electrical motors are machines that perform mechanical operations by converting electrical energy into mechanical energy. These motors are designed to run on alternating current (AC) or direct current (DC). AC motors have two types: Synchronous Motors and Asynchronous Motors. Both of these machines share a few similarities, like their construction, but they are quite different when it comes to functioning and performance.

We are going to discuss the basics of Synchronous motor and Asynchronous motor before we look at their differences.

An induction or AC motor is an asynchronous motor. Induction-motor operation is asynchronous because of the slip due to which the rotation speed of the stator field is a bit slower than the rotor field speed.

The rotor of most induction motors today is called a squirrel cage. The cylindrical squirrel cage is composed of heavy copper, aluminium, or brass bars set into grooves and electrically shorted at both ends. The solid core is constructed of multiple layers of electrical steel laminations. The stator contains more slots than the rotor. The number of rotor slots has to be a non-integral multiple of stator slots so that the teeth of the rotor and the stator do not interlock magnetically when the motor is powered on.

Induction motors can also be found with rotors made of windings rather than squirrel cages. The purpose of this wound-rotor design is to reduce rotor current as the motor begins to rotate. This is accomplished by connecting every rotor winding in series with a resistor. A slip-ring arrangement provides current to the windings. Once the rotor reaches maximum speed, the rotor poles are short-circuited, so it operates like a squirrel-cage rotor electrically.

The stator or the armature of the motor windings is the stationary part of the motor. The AC supply is connected to the stator windings. When a voltage is applied to the stator winding, current starts flowing in the stator. The flow of current creates a magnetic field that affects the rotor, which then sets up voltage and current to flow in the rotor winding.

A north pole in the stator will induce a south pole in the rotor. However, the stator pole rotates when the AC voltage alters in amplitude and polarity. The induced rotor pole attempts to follow the stator pole as it rotates. However, Faraday’s law states that an electromotive force is generated when a loop of wire moves from a low strength magnetic-field to one of high strength magnetic-field, and vice versa. The magnetic field would remain constant if the rotor followed the moving stator pole. Therefore, the rotor field rotation is always lagging behind the stator field rotation. The rotor field always lags and runs behind the stator field. This results in the rotation occurring at a speed that is somewhat slower than that of the stator. The slip is the difference in speed between the two fields.

The amount of slip can be variable. It is mainly affected by the load that a motor drives, and also by the resistance of the rotor circuit and the strength of the field induced by the stator flux.

Explanation of synchronous motors

Synchronous motors use a special rotor construction that allows them to spin at the same speed as the stator field — so the motors are in synchronization with the stator field. Usually synchronous motors are used for applications requiring position control. A good example of a synchronous motor is the stepper motor. Nonetheless, the development of power-control circuitry has led to the development of synchronous-motor designs that are optimized for use in high-power applications, such as fans, blowers, and driving axles in off-road vehicles.

Synchronous motors are of two basic types:

  • Self-excited: Based on the same principles as induction motors,
  • Directly excited: Field mostly with permanent magnets, but not always

Besides being called a switched-reluctance motor, a self-excited synchronous motor also contains a rotor cast of steel that includes notches or teeth, called salient poles. The notches on the rotor let the rotor lock with the stator field and run at the same speed.

In order to move the rotor from one position to the next, successive stator windings/phases must be sequentially switched in a manner like that of stepping motors. Several different names may be used to describe the directly excited synchronous motor. The common names include ECPM (Electronically switched permanent magnet), BLDC (brushless DC), and brushless permanent magnet motor. The rotor in this design contains permanent magnets. The magnets can either be mounted on the rotor surface or inserted into the rotor assembly.

The permanent magnets of this design prevent slippage and are the salient poles. A microprocessor sequentially switches electrical power to the stator windings at appropriate times using solid-state switches, minimizing torque ripples. All these synchronous motors have the same operating principle. Basically, a substantial amount of magnetic flux crosses the air gap between rotor and stator when power is applied to coils wound on stator teeth. The flux crosses the air gap perpendicularly. If the stator and rotor are perfectly aligned, there will be no torque produced. If the rotor tooth is positioned at an angle to the stator tooth, then at least some flux crosses the gap at a non-perpendicular angle to the tooth’s surfaces. A torque is generated on the rotor as a result. Thus, switching power to stator windings at the right moment causes either clockwise or counterclockwise motion, depending on the flux pattern.

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